Laser arrays having a plurality of parallel laser stripes arranged in a single semiconductor so that each stripes acts as a waveguide and the parallel stripes interact for phase locked operation are known. Such arrays usually excite a plurality of supermodes including the fundamental supermode and higher order supermodes. Each supermode has its own distinctive field pattern, and whichever supermode is predominant will have a major effect on the pattern of light emitted by the laser array.
In greater detail, FIG. 2(a) shows a crosssectional view of a prior art semiconductor laser array including five laser stripes laterally spaced from each other in the same serially disposed semiconductor layers. In a laser array like that of FIG. 2(a), the spacing of the stripes and the materials between the stripe grooves establish phase relationships between the radiative oscillations produced by each of the laser stripe. Those phase relationships determine the oscillation modes of the array and its radiation pattern. Because several different phase interactions may occur between the different laser stripes in an array, laser arrays may support more than one oscillation mode, usually called a supermode. Generally, only one of the several possible supermodes is excited when a laser array is operated. That excited supermode generally exhibits the lowest threshold current for laser oscillation, i.e., the highest modal gain of the available supermodes. It is desirable to discriminate among the supermodes and to suppress unwanted supermodes to achieve optimal phase coherence in the radiation produced. Oscillation in the fundamental supermode desirably produces a single-lobed beam directed parallel to the laser waveguides. Higher order supermodes may produce two-lobe far-field radiative patterns. The favored supermode depends upon the lateral gain distribution across the array.
The structure and operation of a known semiconductor laser array employing five uniformly spaced laser stripes is explained with reference to FIG. 2(a). The array includes a substrate 1 which may be p-type GaAs. An oppositely doped, discontinuous current blocking layer 2, such as n-type GaAs, is disposed on substrate 1, preferably being deposited by an epitaxial growth process. In the array of FIG. 2(a), five stripe grooves 3 have been formed by chemically etching through layer 2 and into substrate 1 at five separate locations using a conventional photolithography technique. The etching divides formerly continuous layer 2 into four spaced apart mesas between and defining the grooves 3. Grooves 3 are uniformly spaced with a separation d of about two microns and each groove has a width w of about three microns.
A first cladding layer 4 is disposed in and over grooves 3 and over the mesas and islands of layer 2. Cladding layer 4, in the example described, is p-type AlGaAs. An active layer 5 is disposed on cladding layer 4. In the example being described, active layer 5 is p-type AlGaAs. A second cladding layer 7, in the example n-type AlGaAs, disposed on active layer 5. Layers 4, 5, and 7 are all produced by epitaxial growth, preferably liquid phase epitaxy, although other techniques such as metal organic chemical vapor deposition or molecular beam epitaxy can be used in appropriate circumstances.
Layer 2 functions as a current blocking layer because of the pn junction formed between it and substrate 1. Because of layer 2, currents flowing transversely through the layers are concentrated in the areas of grooves 3. As a result, only the encircled regions 6 of active layer 5, each of which is disposed opposite a respective groove 3, produces radiative oscillations. As in a conventional semiconductor laser, the light oscillations produced in active regions 6 are transversely confined by cladding layers 4 and 7 and are laterally confined by the relatively lossy regions of layer 2. This waveguide confinement results in the desired laser oscillation and radiative emission.
The structure of FIG. 2(a) produces a refractive index distribution of the type illustrated in FIG. 2(b). The refractive index has one effective value at each groove 3, indicated by width w in FIG. 2(b), and a different, lower value in the region between adjacent grooves, indicated as d in FIG. 2(b). In the example described, the change in refractive index between these two regions, .DELTA.n, is about 0.0031. That refractive index distribution results in an electric field distribution, when the laser array is in operation, as indicated in FIG. 2(c). The electric field distribution has five peaks of relatively uniform amplitude and intervening valleys.
In operation, each of the five laser stripes in the array of FIG. 2(a) operates as a single stripe laser. In addition, if the stripes are spaced sufficiently closely, such as the 2 microns spacing of the example, there is interaction between the stripes of the array. The interactions of the radiative emissions produced at each of grooves 3 can support several supermodes of oscillation. The oscillations generally occur in one or more characteristic modes, i.e., one or more eigenmodes, for the array. The relative gain of the separate modes can be determined by measuring and by analyzing the radiative pattern of the array. Generally, there will be a fundamental supermode and higher order, i.e., harmonic, supermodes so that the total number of supermodes produced equals the number of grooves. In keeping with the usual fundamental and harmonic mode numbering scheme, the fundamental supermode number is assigned the number 1, and higher order supermodes are assigned increasing integers.
The relative gain of the various supermodes for the array of FIG. 2(a) is shown in FIG. 3(a) which is taken from Applied Physics Letters, Vol. 55, No. 3, pps. 200-202 (1984). In calculating the relative gains of the various supermodes, it is assumed that the gain distribution is proportional to the square of the electric field intensity distribution of the array.
As shown in FIG. 3(a), as generally experimentally verified, and as further described in the Applied Physics Letters article, the highest order supermode of the array of FIG. 2(a) has the highest relative modal gain and is therefore the predominant supermode, i.e., the one that is supported at the lowest threshold current. That mode produces an undesirable two-lobe far-field radiation pattern. A radiation pattern with more than one lobe is difficult or impossible to collimate in a simple optical system. A single-lobe far-field pattern produced by suppressing the higher order supermodes makes the laser array of FIG. 2(a) more useful. By suppressing the gain of the supermodes having numbers two through five so that the fundamental supermode is predominant or the only supermode, a single-lobe farfield radiation pattern may be produced.
The aforementioned Applied Physics article is said to introduce the concept of nonuniform laser arrays, in which the channels (or stripes) are made purposely nonidentical to discriminate between the supermodes and allow the suppression of all higher order supermodes. Two so-called chirped arrays are described, and both are said to produce gain distributions in which the fundamental supermode predominants. The nonuniform arrays which are described are termed the linearly chirped array and the "v" chirped array.
In a five laser array having an inverted "v" chirped distribution, the groove widths are symmetrically distributed with the widest groove in the center, adjacent narrower grooves and still narrower outside grooves. The resulting refractive index and electric field distribution for the inverted v chirped groove width distribution are shown in FIGS. 4(a) and 4(b), respectively. That electric field distribution is sharply peaked at the center and has substantially suppressed side lobes, as indicated by the relative amplitudes illustrated in FIG. 4(b). As a result, when the laser is operated there is a concentration of energy at the central peak which establishes a limit at which the laser can be operated; operation beyond that limit can cause localized failure at the part of the array associated with the central peak because of the energy concentration at that peak which is achieved by the nonuniform spacing.
Another chirped groove distribution disposes the narrowest groove at one end of the array with increasing groove widths to the widest groove at the opposite end of the array. The resulting refractive index and electric field distributions (see relative amplitudes illustrated in FIG. 5(b)) for that groove distribution are shown in FIGS. 5(a) and 5(b), respectively. The electric field distribution includes an amplitude peak at the end of the array which has the widest groove. Like the v-chirped array, the linearly chirped array also has a sharp peak in energy distribution which results in the potential for laser failure at the part of the array associated with the energy peak.
It should be noted that the diagrams of FIGS. 4(b) and 5(b) represent electric field intensity plotted against position on the array. Because the energy emitted from the laser depends upon the square of the electric field intensity, the concentration at a local area of the facet is even stronger than indicated by the electric field distributions of FIGS. 4(b) and 5(b). These relatively high concentrations of energy limit the power that can be produced by the laser arrays without exceeding safe operating temperatures at local areas on the radiating facet.